It has been generally known that a thermoplastic resin can be flame retarded by adding a flame retarder or flame retarder and flame retarding assistant such as an organic halide, organophosphorus compound, antimony oxide, etc., to the resin. However, by examining individual thermoplastic resins, it has been found that the attained degree of flame retardation is different for each resin due to the difference in its properties, and flame retardation is not necessarily attained in a state suitable to the utility of each resin. Among various types of resins, it is extremely difficult, especially with a polyolefin, to make it flame retarding and to maintain its good properties.
With regard to the attainment of flame retardation for a polyolefin, it is already known to be possible to some degree by adding the aforesaid flame retarder or flame retarder and flame retarding assistant. It is also general knowledge that talc is effective to prevent fusion dropping at combustion, which is possible to be a source for second ignition for a fire (Japanese Patent Publication Gazettes No. 32288/1972 and 6105/1972, Japanese Patent Gazette laid open to public 345/1974, etc.). On the other hand, the industries relying upon electrical machinery and appliances demand thin moldings of polyolefins because of their good electrical properties. In addition, burning resistance is lowered and fusion dropping at combustion occurs more easily as the moldings are made thinner. However, the aforesaid techniques cannot afford high flame retarding properties without fusion dropping in the case of thin moldings of polyolefins. Recently, a method for making a device in diffusion state of flame retarder as the measure for affording higher flame retarding properties to plastics, mainly polyolefins, was reported (Japanese Patent Gazettes laid open to public 14946/1976 and 39739/1976). According to this method, however, thermal hysteresis of extremely high temperature (higher than 290.degree. C.) is caused at the time of manufacturing and it damages the properties of plastics. Therefore, this method is impractical for actual application.
As a consequence of studies made to develop compositions with high flame retarding properties without fusion dropping in thin moldings of polyolefins, while maintaining good properties of the polyolefins, we have found unexpectedly that by combining polypropylene with polyethylene at specific ratios, not only are physical properties maintained but also flame retarding properties are improved.